1. Field of the Invention
The present invention relates generally to a pressure regulating system for use in a hydraulic control system for an automatic transmission and more particularly to a pressure regulating system for an automatic transmission being provided with a belt drive continuously-variable speed transmission mechanism.
2. Description of the Prior Art
A belt drive continuously-variable speed transmission mechanism is combined with a torque converter or a fluid coupling, and a changeover mechanism including a forward drive and a reverse drive to form an automatic transmission for a vehicle. Such an automatic transmission is controlled by a hydraulic control system which controls the supply of working fluid to the hydraulic servomotors of the belt drive continuously-variable speed transmission mechanism and the changeover mechanism according to input signals corresponding to the running conditions of the vehicle, such as running speed and the degree of throttle opening. The hydraulic control system includes a pressure regulating system which produces line pressure corresponding to above-mentioned input signals. In a conventional pressure regulating system, the running speed and the degree of throttle opening are detected mechanically by means of a governor valve or the like or electrically and the detected results are converted into oil pressures which are used as input oil pressures for the pressure regulating system. Accordingly, it has been difficult to apply an appropriate line pressure corresponding to the variation of the transmission torque or the reduction ratio of the belt drive continuously-variable speed transmission mechanism to the servomotors thereof.